Transcription

2 What does the night sky look like? The disk of the Milky Way, our galaxy

3 M81: a spiral galaxy like the Milky Way A galaxy is made up of billions of stars, dust, gas, and lots and lots of dark matter (which we don t understand at all!) G. Benintende/APOD

4 M81: a spiral galaxy like the Milky Way If this were the Milky Way, here is where the Sun would be G. Benintende/APOD

5 The Big Bang happened everywhere 13.8 billion years ago The history of the universe Now: the present epoch

6 From There to Here: How can we know this? L. Comolli (APOD) time Center for Cosmological Physics U Chicago

7 Cosmological models How did the structures we see today form and evolve? Do our models predict this behavior? Can they give us any insight into what is going on? Is there enough time for stars/galaxies to form? time

8 How old is the universe? Light travels at a finite speed! 186,000 miles per second or 300,000 kilometers per second or 670,616,629 miles per hour When we view a distant galaxy, we see light that left that galaxy billions (and billions) of years ago. Observing the distant universe allows us to observe the universe at an earlier age in its history. Hubble extreme Deep Field NASA The universe must be older than the light travel time of the most distant object we can see.

9 Looking out is looking back The time it takes light to travel from a distant object to us depends on the geometry of the universe longer path shorter path In order to know the light travel time, we must know the geometry The geometry depends on the amount of matter (baryonic and dark) and dark energy(!)... A quick review

10 What is the Geometry of cosmic space? It depends on the density of its matter+energy contents A high density Universe has POSITIVE curvature A low density Universe has NEGATIVE curvature A Universe with zero curvature, said to be FLAT, has critical density

12 History and Fate of the Universe Recollapsing Universe: the expansion will someday halt and reverse Critical Universe: will not collapse, but will expand more slowly with time Coasting Universe: will expand forever with little slowdown Accelerating universe*: Expansion will accelerate with time *Currently favored but the one with the dark energy problem! (more about this in the next lecture)

13 Planck satellite (mission in progress 2014) In the very early universe (The Big Bang was hot!), the temperature and density were very, very high too high for atoms to exist. Subatomic particles existed, the protons and electrons were free, and photons carried most of the energy. But the photons scattered off the free electrons before they travelled very far (and so they never reach us). As the universe expanded, it cooled. About about 400,000 years after the BB, the universe had cooled to a temperature of about 3000 degrees Kelvin, cool enough that protons and electrons could combine to form hydrogen. The formation of atoms released photons according to Wien s Law (and a temperature of 3000 K) We observe those photons REDSHIFTED (z ~ 1000) => T= 3 K The sudden decline in free electrons removed the source of scattering: the photons are seen today!

14 2013+: Planck Satellite Planck satellite map of the CMB photons Colors indicate temperatures a bit hotter than or colder than 3 K. Amount of difference in CMB temperature and over what scale it occurs gives us a snapshot of the universe at age 400,000 years AND

15 Planck CMB map: a work in progress The Cosmic Background Radiation as revealed by Planck details tells us about the geometry of the universe, i.e. its CURVATURE Dark spots are real More precision in parameters A work in progress (and recent controversial results; a subject for a meal!)

16 Between emission of CMB and formation of first stars, galaxies and black holes, there were no sources of light. Dark Ages The hydrogen gas was atomic (rather than ionized). The universe was dark

17 First sources of light form (First stars? Galaxies? Black holes?) When they start to emit photons, the hydrogen atoms get ionized again. The universe lights up! Cosmic Dawn Epoch of Reionization

18 Epoch of Galaxy and Star Formation First sources of light form (First stars? Galaxies? Black holes?) At about the same time, the first galaxies formed. Which formed first? Stars? Galaxies? Black holes? And how did we get the galaxies, clusters and superclusters that we see today?

19 A brief history of the universe

20 Fate of the Universe: What are the options? The curvature of the universe determines the path light follows through it, and hence how long it takes a photon to reach us from a distant object (the light travel time or lookback time ). Energy of Expansion versus Gravitational Energy What will win this tug of war? How much matter does the universe contain? (More tomorrow )

21 A brief history of the universe Add in the FUTURE

22 21 st century cosmology The Big Bang occurred everywhere in the universe 13.8 billion years ago. The expansion of the universe today is accelerating. As far as we can tell the geometry of the universe is flat (Euclidean) because the universe is close to critical (matter only). Therefore, the universe will expand forever. There are still fundamental but unanswered questions! And there are real PROBLEMS!... More on this tomorrow.

23 Looking out is looking back but it isn t easy! Distant galaxies are very FAINT and very SMALL and very RED! Launched 2003 NASA/JPL-Caltech/STScI/UTokyo Launched 1990

24 Measuring extragalactic distances Primary distance methods: Measure distance to some standard candle (object whose luminosity you can infer) within the galaxy Works for nearest systems (where e.g. we can resolve the individual stars) or, in very special cases, also for distance systems. Hubble s Law: Observationally low-cost (but not exactly cheap ) Measure the redshift => measure the distance Need to worry however about orbital motions in groups/clusters (and other similar effects)

25 Early History of Hubble s Law Slipher (~1912) noticed that spiral nebulae showed almost predominantly redshifts. By 1925 he had radial velocities for 40 galaxies Hubble used the 100-inch telescope on Mt. Wilson to measure distance to 18 galaxies Found linear relation between increasing redshift and increasing distance, now known as Hubble s law H o d = v ~ cz Of course: A major goal of HST has been to measure H o ~ 70 km/s/mpc

26 Edwin Hubble ~ Hubble s Law Discovered first Cepheids in M31 and M33 VERY DISTANT Measured distances to 18 galaxies for which Doppler shifts also measured. The further away a galaxy is from us, the faster it is moving away from us. Velocity away from us Hubble s Law Distance from us Hubble s law works if the only velocity a galaxy exhibits is the expansion of the universe; galaxies in groups/clusters also have velocities due to their orbits in the group/cluster.

27 Cepheid Variables Cepheid variables: Analogs of the star Ceph Giant, post-main sequence stars which pulsate due to instabilities. The brightness changes, in a periodic way

28 Cepheid Period-Luminosity Relation Henrietta Leavitt discovered a relationship between the period of pulsation and the mean luminosity of the star. 1. Identify the star as a Cepheid variable by studying its spectrum (if possible) and/or by the shape of its lightcurve. 2. Calculate its period. 3. Use the Period-Luminosity relationship to determine the Luminosity. 4. Use the inverse-square law to calculate how far a star of that luminosity would have to be in order to appear as a star of that observed apparent brightness.

29 Supernova Type Ia in M51

30 Using SNe to determine distances What measurements would you make? Observe the supernova suddenly brighten, then fade over time. Identify it as a SN of Type Ia from its light curve (how it brightens and fades) and probably from taking spectra (which shows heavy elements like chromium, aluminum, etc) What assumptions would you need to invoke? That all SN of Type Ia have the same luminosity when they are at their maximum apparent brightness, i.e. that SNeIa are standard candles. So, if we observe a supernova s light curve and apparent brightness, we can derive its distance. Then we can compare that distance to the distance we expect from its redshift and Hubble s law => any differences tell us about the geometry of the universe.

31 Determining distances from SNe What measurements would you make? Observe the supernova suddenly brighten, then fade over time. Identify it as a SN of Type Ia from its light curve (how it brightens and fades) and probably from taking spectra (which shows heavy elements like chromium, aluminum, etc) What assumptions would you need to invoke? That all SN of Type Ia have the same luminosity when they are at their maximum apparent brightness. That you can properly account for dust obscuration within the galaxy in which the SN resides. How do you derive the distance to a galaxy with a SNeIa? Now we have: luminosity and apparent brightness. Remember that: Apparent brightness Luminosity (Distance) 2 => distance

33 How old is the universe? The universe must be older than the oldest stars. Hydrogen and helium were made in the first 3 minutes of the universe; heavier atoms are manufactured in stars and/or in stellar explosions. More massive stars burn their fuel faster and hotter than low mass stars (like the Sun). More massive stars live much shorter lifetimes than low mass stars (like the Sun). Not all stars were born at the same time; later generations of stars (like the Sun) contain more heavy elements to start with than do earlier generations. Generations of stars must have lived and died to produce the heavy elements (carbon, nitrogen, oxygen, iron, aluminum etc) which we find on Earth and in our bodies today.

34 Where does the carbon come from? The Sun contains, by mass, 74.9% Hydrogen and 23.8% Helium. The heavier elements make up less than 2% of the Sun s mass. We are a carbon based life form. O C H Earth s atmosphere

35 We are stardust! Only Hydrogen and Helium where made during the Big Bang era. The other elements are manufactured by stars (in different phases of their evolution). The carbon in our bodies was manufactured in previous generations of massive stars, before the Sun was formed: we are stardust!

36 How old is the Universe? If the universe were much younger than 13.8 billion years, there would be more problems: The Sun needs to exist! The Milky Way Galaxy needs to exist! The heavy elements need to exist! The Anthropic Principle: The universe is the way it is because if it were different, we would not be here. If we can measure how far away galaxies are, we can learn about the geometry of the universe and how and how long it has been expanding.

37 The Anthropic Principle The weak anthropic principle: The universe must be compatible with our existence. The strong anthropic principle: The universe is as it is because its purpose is to create life Only universes where we exist are allowed Example: A quick Big Crunch universe that never developed life (carbon based) could not be our universe We live in a golden age If the universe were only 1 billion years old, there would not have been enough time to build up carbon, small rocky planets would not have formed. If the universe were 100 billion years old, most stars would have evolved off the MS and become white dwarfs; we would not see any young stars

38 The Anthropic Cosmological Principle The Anthropic Cosmological Principle J. Barrow and F. Tipler (1986; Oxford U. Press) The weak anthropic principle: The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirements that the Universe must be old enough for it to have already done so. The strong anthropic principle: The Universe must have those properties which allow life to develop within it at some stage in its history.

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